Thallium is highly toxic to humans and other mammals (ToxFAQs: Thallium)
and several radioactive thallium isotopes are known. In microorganisms, thallium is known to inhibit growth and to be accumulated
via potassium transport systems
(Norris et al, 1976; Damper et al, 1979).
Methylated thallium species have been detected in natural waters and production
of dimethylthallium by anaerobic microorganisms in lake sediment has been reported (Schedlbauer and Heumann, 2000.)